Non-contact communication systems, such as FeliCa (a registered trademark of Sony Corporation), are widely used today. Such non-contact communication systems are employed in the ticket gate systems for public transportation such as trains and buses, and the electronic money systems that can be used at various kinds of shops and vending machines.
FIG. 1 shows an example structure of a conventional non-contact communication system. This non-contact communication system 10 is formed with a reader/writer 11 and a transponder 12. In a case where this non-contact communication system 10 is employed at a ticket gate of a train station, for example, the reader/writer 11 is installed in the ticket gate, and the transponder 12 is embedded in an IC card serving as a ticket, such as Suica (a trademark).
In a case where predetermined transmission information is transmitted from the reader/writer 11 to the transponder 12, a sinusoidal carrier signal (a carrier wave) like the signal shown in A of FIG. 2 is ASK (amplitude shift keying)-modulated and transmitted in accordance with the transmission information. In a case where predetermined digitized response information like the information shown in B of FIG. 2 is transmitted from the transponder 12 to the reader/writer 11, on the other hand, load modulation is performed to cause changes in the voltage of the carrier signal as shown in C of FIG. 2 by switching on and off a damping resistor R1 in the transponder 12 with a switch in accordance with the predetermined response information (see Patent Document 1, for example).
The load-modulated carrier signal is then received by an antenna of the reader/writer 11. The received load-modulated carrier signal has its degree of modulation lowered in accordance with the distance between the reader/writer 11 and the transponder 12, as shown in D of FIG. 2. As the degree of modulation becomes lower, detection of the response information becomes more difficult.
In a case where the non-contact communication system 10 is used in a ticket gate at a train station or the like, to ensure user friendliness, it is required that communication should be possible even if the distance between the reader/writer 11 and the antenna of the transponder 12 is 10 cm or longer.
If the distance between the reader/writer 11 and the transponder 12 becomes longer, the degree of modulation of a load-modulated carrier signal becomes lower, and detection of response information becomes difficult as described above. To compensate for that, the peak-to-peak voltage (hereinafter referred to as Vpp) of the carrier signal is increased to approximately 20 V.
FIG. 3 shows an example structure of a conventional reader/writer that can be used in a case where Vpp of the carrier signal is 20 V. This reader/writer 20 detects response information based on amplitude changes in the voltage of a load-modulated carrier signal.
In the reader/writer 20, full-wave rectification is performed on the load-modulated carrier signal, and envelope detection is performed on the carrier signal by a peak holding circuit that outputs a detection signal. For example, in the case of a carrier signal load-modulated to a modulation degree of 10% as shown in A of FIG. 4, full-wave rectification is performed as shown in B of FIG. 4, and a detection signal with a 1 V difference is output as shown in C of FIG. 4.